Усилители мощности стерео — Musical Fidelity NU-VISTA PAS

The Nu-Vista PAS is a fully balanced, discrete stereo power amplifier from Musical Fidelity, built as a two-chassis design with a separate power supply unit. It develops the circuit of the earlier Nu-Vista 800, taking the Nu-Vista range fully balanced for the first time, and sits in the company's PRE, PAS and PAM separates system alongside the matching Nu-Vista PRE preamplifier and the higher-power Nu-Vista PAM monoblocks.

Key specs that matter

Output is more than 300W into 8 ohms, 500W into 4 ohms and 1,000W peaks into 2 ohms, with an output voltage of 49V RMS and 130A peak-to-peak current. The input stage uses eight 6S51N nuvistor tubes, four per channel, and the output stage has five pairs of thermally compensated output devices per channel. THD is typically below 0.003% from 20Hz to 20kHz, signal-to-noise ratio is above 108dB (A-weighted), channel separation is over 96dB and frequency response is +0/-1dB from 5Hz to 50kHz. Gain is 16x (23.5dB), input sensitivity for full output is 3V RMS and the damping factor is 200. Each chassis measures 483mm wide and 187.5mm high, with the amplifier weighing 27kg and the power supply 40kg.

Engineering and use context

Nuvistors are miniature metal-and-ceramic tubes developed in the late 1950s and used in studio equipment, noted for high reliability, low microphony, low noise and consistency between batches. In the PAS they operate in a discrete Class A buffer at the input, followed by a balanced configuration circuit and the balanced-input output stage. That output stage is based on Musical Fidelity's 1,000W Titan, with separate capacitors for the bulk supply plus a dedicated capacitor for each output device to lower output impedance. Chokes are added to the supply to reduce ripple and current spikes. The bias is set for more than 5W of Class A operation before the stage slides into Class B, at a point where the switching discontinuity is smaller than ordinary crossover distortion. Housing the transformers in the external PSU keeps their magnetic fields away from the sensitive amplifier circuitry. A dimmable colour display can show the selected input or optional VU meters.

Pairing

Four independent inputs (two RCA and two XLR, with fully balanced circuitry on the XLR inputs) and two loop-through outputs (one RCA, one XLR) allow the PAS to be combined with any preamplifier and used for bi-amping. Two pairs of speaker terminals accept 4mm plugs, and 3.5mm trigger input and output sockets switch it with the rest of the system. The matching partner is the Nu-Vista PRE.

Alternatives

Musical Fidelity's own Nu-Vista PAM monoblocks offer higher per-channel power in separate chassis for the most demanding loudspeakers.